The Green Lakes Nature Reserve is one of the most valuable and picturesque landscapes in the Verkiai Regional Park. A number of tunnel valley lakes can be found in two deep, winding and narrow tunnel valleys of oblong shape. These tunnel valleys were formed by the water of the melting glaciers around 18 thousand years ago. The Green Lakes are a mesmerizing sight due to the color of their water which is green. This was how the lakes got their name. The green color of the water is caused by the large quantities of melted carbonates (limestone) in the groundwater that feeds the lakes.
Lake Balsys going deep beneath the tunnel valley is of special interest. Its woody shores rise up to 40 m above the water level but also go deep beneath the surface (the deepest spot of the lake reaches 39 m). Lake Balsys, also known as Lake Kryžiokai (Crusader Lake), is the deepest and largest lake among all the Green Lakes. Legends say that in the times of Vytautas the Great, the beaten Teutonic Order was retreating across the lake. They got surrounded, the ice gave in and the crusaders drowned to the very bottom. For many years, the local fishermen had been fishing out armor and guns in their nets.
The length of Lake Balsys is about 2 km and its width is up to 0.5 km. The water of the lake remains green no matter the season and even when it is iced over. A picturesque trail of 7 km in length goes around the lake. An observation deck is located near the Lake Balsys beach providing a great view of the lake and the surrounding forests.
Lake Gulbinas and Lake Mažasis Gulbinas also belong to the Green Lakes and are situated in another tunnel valley. Compared to Lake Balsys, Mažasis Gulbinas is high up, almost 20 m higher than Balsys. Its shores are lower and the lake itself is not as deep. Mažasis Gulbinas is connected to the longer though shallower Lake Gulbinas by a narrow channel. The latter lake has a small woody island with an ancient hillfort. The Riešė Creek flows into the middle of the lake and exits mere 100 m away.
Lake Akis is shallow and one of the smallest Green Lakes. Measuring 0.5 ha in area, it is located in the same tunnel valley as Lake Balsys. The small Lake Raistelis and Lake Baraukos Akis are located near Lake Gulbinas.
The magnificent hilly lands between the lakes are valuable in terms of both the landscape and biology. This territory is home to many plants recorded into the Lithuania Red Book of Threatened Species and mature linden, oak and fir groves that contain one of the largest populations of cypripedium calceolus in Europe. This area (133 ha) was announced a territory of European conservation significance that, apart from cypripedium calceolus, has several other highly protected plant communities. The Green Lakes also contain quite a few rare and protected plants. A type of green algae called lychnothamnus barbatus is the rarest plant community of all propagated in the depth of 0.5-3 m in the southern part of Lake Balsys. Swamp sawgrass growing at the borders of the lakes amongst bulrushes, reeds and sedges is relict plant and very rare both in Lithuania and the neighboring countries.